Added HTTP status code to parsed response

This is very useful for somebody who wants to do their own error
handling. In my case I turned off  throws_exception and was handling
the error response from MailChimp directly and I had to know the status
code before knowing how to proceed with the error.
Modified Test cases to replicate actual scenarios

expect_post method changed to expect_post_success and
expect_post_internal_error with each method returning appropriate HTTP
status codes and responses.

Also, the and_return block for the post methods now returns a JSON hash
instead of an empty array in some cases. This is what happens in real
world scenarios too.
Return status code only if body is present
Merge pull request #56 from nazmir/master

Added HTTP status code to parsed response